Method for automatically adjusting the filter parameters of a digital equalizer and reproduction device for audio signals for implementing such a method

ABSTRACT

A method is proposed for automatically adjusting the filter parameters—center frequency, quality and amplification or attenuation—of at least one digital equalizer ( 6,7 ) which is a component of a reproduction device ( 1 ) for audio signals in a vehicle passenger compartment.  
     To that end, first of all, the acoustical frequency response of the passenger compartment is ascertained. The inadequacies in the acoustics of the passenger compartment in the form of local maxima and minima in the measured frequency response are then determined . On this basis, the filter parameters are adjusted automatically so that at least a portion of these inadequacies is compensated.  
     A reproduction device for audio signals for implementing this method is also proposed.

BACKGROUND INFORMATION

[0001] The present invention relates to a method for automaticallyadjusting the filter parameters—center frequency, quality andamplification or attenuation—of at least one digital equalizer which isa component of a reproduction device for audio signals in a vehiclepassenger compartment. The invention also relates to a reproductiondevice for audio signals for implementing such a method, having aloudspeaker device and having an audio processor which includes at leastone digital equalizer, is arranged in the signal path between at leastone signal source and the loudspeaker device, and is connected to acontrol processor via a control bus.

[0002] The present invention starts from the car radio devices, knownfrom practice, which are based on the so-called 2-IC technology. Inthese car radio devices, two or three freely programmable audio filtersare integrated into the signal path. These digital parametric equalizers(DPE) are available to the user to compensate for acousticalshortcomings in the passenger compartment. The user is able to vary eachfilter with respect to center frequency, quality, i.e. filter width, andamplification or attenuation, in order to compensate for excessive risesand so-called holes in the acoustical frequency response of thepassenger compartment.

[0003] However, this proves to be problematic in practice, since theuser must know the acoustics of his/her vehicle very well to optimallyadjust the equalizers, and it is very difficult to ascertain theacoustical frequency response solely by listening, without metrologicalaid. The operating instructions of the known car radio devices are onlyable to provide very limited assistance for the best possible adjustmentof the equalizers, since on no account is it possible to consider alltypes of vehicles here, and by no means the great number of individuallayout variants, as well as loudspeaker and amplifier configurations.

[0004] Moreover, car radio devices are known having an audio module,integrated in the signal path, on which a graphic equalizer isimplemented with the aid of a digital signal processor. The seven ornine bands of such a graphic equalizer are fixed in their centerfrequency and quality, and are only variable in their amplification. Theseparate audio module of these car radio devices permits an automaticcalibration of the graphic equalizer. To that end, the acoustics in thepassenger compartment are measured with the aid of a microphoneconnected to the audio module via an analog-to-digital converter. Usinga special software, the graphic equalizer is subsequently adjusted insuch a way that the inadequacies of the acoustics are compensated for inthe best way possible.

[0005] The use of a graphic equalizer to compensate for the inadequaciesin the acoustics of a passenger compartment proves to be problematic inpractice. As already mentioned, the center frequencies of the equalizerbands of a graphic equalizer are fixed. As a rule, they are spaced apartby a minimum of one octave in the case of nine bands. Thus, it is notpossible to optimally compensate for narrow resonance rises, which liebetween the equalizer bands, in the acoustical frequency response of thepassenger compartment. Moreover, the additional audio module having thedigital signal processor for implementing the graphic equalizer and forcalibrating this equalizer is relatively cost-intensive.

SUMMARY OF THE INVENTION

[0006] With the present invention, it is now proposed to adjust thefilter parameters—center frequency, quality and amplification orattenuation—of the digital equalizer(s) automatically, in order torelieve the user of the difficult task of adapting the digitalequalizer(s) to the special acoustics of his/her vehicle passengercompartment.

[0007] This is achieved according to the present invention by a methodfor automatically adjusting the filter parameters, in which first ofall, the acoustical frequency response of the passenger compartment isascertained, then the shortcomings in the acoustics of the passengercompartment in the form of local maxima and minima in the frequencyresponse are determined, and thereupon the filter parameters areadjusted automatically so that at least a portion of these shortcomingsis compensated for.

[0008] Moreover, a reproduction device of the type indicated at theoutset is proposed which, according to the present invention, toautomatically adjust the digital equalizer(s), includes a noisegenerator, via which a noise signal may be supplied to the equalizer. Inaddition, the control processor includes means, via which the filterparameters are adjustable so that the equalizer has a bandpasscharacteristic with a narrow bandwidth, the center frequency beingvariable over the audio spectrum. To capture the signal emitted by theloudspeaker device into the passenger compartment and to determine thefrequency response, at least one microphone having evaluation means isprovided. Finally, the control processor also includes means via whichthe filter parameters are adjustable, taking into account the measuredfrequency response.

[0009] According to the present invention, it has become known that anautomatic adjustment of the filter parameters of the digital equalizersof a reproduction device for audio signals in a passenger compartment isuseful, since when optimizing the filter parameters, it is necessary toconsider the individual acoustical properties of the passengercompartment, arranged and equipped specific to the user, and theseproperties may be detected best using metrological means. By varying notonly the amplification and attenuation, respectively, of the equalizers,but also the center frequencies and qualities, it is possible tocompensate for the shortcomings in the acoustics of the passengercompartment very well, regardless of the position and the width of theexcessive rises and holes in the measured frequency response.

[0010] Furthermore, it has become known according to the presentinvention that the equalizers to be calibrated, because of theirprogrammability, may be used first of all for determining the acousticalfrequency response of the passenger compartment before the filterparameters are adjusted to compensate for the inadequacies in themeasured frequency response. It has also become known that the filterparameters may be optimized with the aid of a suitable additionalsoftware of the control processor, present anyway, of the car radiodevice. Thus, all in all, no additional audio module having a digitalsignal processor is necessary within the framework of the presentinvention, but rather only a microphone amplification and rectificationcircuit which is coupled to the analog-to-digital converter present inthe control processor. In this manner, only a very small additionaloutlay for hardware and software, and therefore costs, is necessary forthe automatic adjustment of the filter parameters proposed in thepresent invention.

[0011] In principle, there are various possibilities for determining theacoustical frequency response of the vehicle passenger compartmentwithin the framework of the method according to the present invention.In one advantageous variant, the loudspeaker device of the reproductiondevice is triggered in succession by bandpass noise signals havingdifferent center frequencies. The frequency bands, set in each case inthe form of a bandpass noise signal, cover the entire audio spectrum.The frequency response to be determined is now ascertained in the formof frequency measuring points for the individual frequency bands. Thesound level of the signal which, in this case, is emitted by theloudspeaker device into the passenger compartment, may simply bedetermined as a frequency measuring point for a specific frequency band.

[0012] In view of minimizing the hardware and software expenditure, itproves to be advantageous to generate the bandpass noise signals forascertaining the acoustical frequency response of the passengercompartment using the equalizer to be adjusted itself. Since both thecenter frequency and the quality of the equalizer are freelyprogrammable, the filter parameters may be adjusted so that a bandpasscharacteristic having a narrow bandwidth at a predefined centerfrequency results for the equalizer. From a noise signal supplied to it,the equalizer then generates the desired bandpass noise signal or asuccession of bandpass noise signals which cover the entire audiospectrum.

[0013] In principle, there are also various possibilities within theframework of the method of the present invention for the automaticdetermination and adjustment of the filter parameters. In oneadvantageous variant, a plurality of normalized equalizer curve patternsof different quality are stored for this purpose. To determine thefilter parameters, for each curve pattern and each local maximumdetermined in the measured frequency response, the center frequency ofthe curve pattern is now shifted to the local maximum, and anattenuation is determined by scaling the curve pattern to the level ofthis local maximum. The filter corresponding to this scaled curvepattern is then used on the measured frequency response, and thedeviation of the resulting frequency response from a target frequencyresponse is determined. In this way, for each potential center frequencyof the equalizer, as many error values for the deviation from the targetfrequency response are determined as there are curve patterns orqualities stored. The filter parameters—center frequency, attenuationand quality—of that curve pattern for which the smallest error value hasbeen determined are finally taken as the basis for the automaticadjustment of the equalizer.

[0014] In view of the different perception of resonances and holes inthe frequency response, as well as the general dependence of theperception on the frequency of the audio signal, it is advantageous toweight the individual deviations when determining the deviation of afiltered frequency response from the target frequency response. In sodoing, it proves to be useful to weight positive individual deviationsmore strongly than negative individual deviations, so that any remainingexcessive rises in the frequency response are evaluated as worse thanthe holes which are far more uncritical psychoacoustically.Alternatively or in addition thereto, psychoacoustically criticalfrequency ranges may be weighted more strongly than psychoacousticallyuncritical frequency ranges.

[0015] Moreover, it is advantageous if, when determining the deviationof a filtered frequency response from the target frequency response, thelevel of the local maximum or the resonance corresponding to it is takeninto account, so that narrow, high resonances lead to a smaller errorvalue compared to wider, less high resonances, and therefore arepreferably eliminated.

[0016] If the filter parameters of a plurality of digital equalizersmust be adjusted automatically, it is advantageous to determine thefilter parameters of the individual equalizers in succession, in that ineach case, prior to determining the filter parameters of one equalizer,the equalizer(s) adjusted before are used on the measured frequencyresponse.

BRIEF DESCRIPTION OF THE DRAWING

[0017] As already discussed in detail above, there are variouspossibilities for advantageously developing and further refining theteaching of the present invention. To that end, reference is made on onehand to the claims following Claims 1 and 10, and on the other hand, tothe following description of an exemplary embodiment of the inventionwith reference to the Drawing.

[0018] The single FIGURE shows the block diagram of a reproductiondevice for audio signals for implementing the method of the presentinvention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0019] Reproduction device 1 shown in the single FIGURE is used forreproducing audio signals in a vehicle passenger compartment; the audiosignals may be generated by different audio sources 2, 3, such as radio,CD, CC, etc. Reproduction device 1 includes a loudspeaker device 4 andan audio processor 5 that is arranged in the signal path between audiosources 2, 3 and loudspeaker 4 and that has two freely adjustabledigital equalizers 6, 7, via which the signals from different audiosources 2, 3 are fed to loudspeaker device 4. Of course, more than twoequalizers may also be provided here. To adjust the filterparameters—center frequency, quality and amplification or attenuation—acontrol processor 8 sends suitable filter parameters via a control bus 9to audio processor 5.

[0020] To determine the frequency response of the passenger compartment,reproduction device 1 also includes a noise generator 10, via which anoise signal may be supplied to equalizers 6, 7. Noise generator 10 isimplemented here as additional software in audio processor 5, and, ifnecessary, may be started via control processor 8. Alternatively, thenoise signal could also be generated by an external noise source asadditional audio source, for example, with the aid of an appropriate CDor a suitably adjusted tuner.

[0021] Control processor 8 also includes means via which the filterparameters may be adjusted in such a way that equalizers 6, 7 have abandpass characteristic with a narrow bandwidth, i.e. with a quality onthe order of magnitude of 8, the center frequency being variable overthe audio spectrum. In this way, with the aid of noise generator 10 andvia equalizers 6, 7, loudspeaker device 4 may be triggered by a bandpassnoise signal.

[0022] When the calibration of equalizers 6, 7 has been started, forexample, by a keystroke, control processor 8 varies the filterparameters in defined time sequence, so that the center frequency of thebandpass filter decreases, for example, in the one-third-octave intervalfrom the highest to the lowest frequency to be adjusted. The signals,which are then emitted in each case via loudspeaker device 4 into thepassenger compartment, are detected with the aid of a microphone 11 andevaluated by suitable evaluation means 12 for determining the frequencyresponse of the passenger compartment. To that end, the signals sensedby microphone 11 are amplified in an operational amplifier circuit,subjected to a logarithmic procedure and rectified, so that a directvoltage is present at the output of this circuit. The magnitude of thisdirect voltage is proportional to the sound level or sound pressure inthe passenger compartment for the frequency band, which is adjusted bythe respective bandpass noise signal. The sound level for the entireaudio spectrum is detected by the tuning of equalizers 6, 7.

[0023] The direct voltage representing the sound level is sampled by ananalog-to-digital converter 13 of control processor 8, so that after thetuning of all frequencies or frequency bands to be measured with thecorresponding voltage values, a precise image of the acousticalfrequency response of the passenger compartment is available to controlprocessor 8. The absolute frequency response value or amplituderesponse, and not the phase response, is designated exclusively here asthe frequency response.

[0024] Control processor 8 now ascertains the inadequacies, i.e. theresonances and holes, in the acoustics of the passenger compartment inthe form of local maxima and minima in the measured frequency response,and determines the filter parameters—center frequency, amplification andquality—of equalizers 6, 7, so that these inadequacies are compensatedfor as well as possible.

[0025] The total additional expenditure compared to a car radio devicewhose equalizers are not adjustable automatically is in an additionalhardware 10 or additional software for generating a noise signal, anadditional software in control processor 8 which takes over thesequencing control of the calibration process as well as theascertainment of the best filter parameter setting, and an additionalhardware 12 for the amplification, logarithmation and rectification ofthe microphone signal.

[0026] To ascertain the best possible setting of the filter parameters,normalized equalizer curve patterns having different quality are storedin audio processor 5.

[0027] In one advantageous variant of the method according to thepresent invention, first of all the resonances, i.e. the local maxima,in the frequency response, measured and adjusted by the frequencyresponse of the microphone, are determined. For each curve pattern andeach of these local maxima, the following work steps are then carriedout:

[0028] The center frequency of the curve pattern is shifted to the localmaximum and scaled using the level of the resonance, i.e. the level ofthe maximum. The frequency response resulting therefrom is subtractedfrom the measured frequency response, which corresponds to the use of afilter having the properties of the shifted and scaled curve pattern onthe measured frequency response.

[0029] The deviation of the resulting frequency response from apredefined target frequency response is then ascertained. As a rule, thetarget frequency response is linear, but a raising or lowering ofcertain frequency ranges may also be provided. The deviation isascertained by weighted summation of the amounts of the individualdeviations at the frequency points, and is a measure for how good theequalization is for the individual shifted and scaled curve patterns.The greater the value of the deviation, the poorer the equalizing.Positive deviations are weighted double compared to negative deviations,so that any remaining excessive rises in the frequency response areevaluated as worse than the psychoacoustically far less critical holes.A different weighting of individual frequency ranges is also conceivablehere, since resonances in certain frequency ranges are more criticalthan in others. The result of this weighted summation corresponds inprinciple to the “area” between the target curve and the real curve, theportion above the target curve being evaluated double. An error valuenow exists for each curve pattern, i.e. for each quality, and for eachlocal maximum in the measured frequency response.

[0030] The level of the respective resonance, i.e. of the correspondingmaximum, is also subtracted from this error value. Smaller error valuesare thereby allocated to narrow high resonances, than to wide, less highresonances having the same “error area”. The former are thus preferablyeliminated, which is useful from the psychoacoustical standpoint.

[0031] For each potential equalizer center frequency, as many errorvalues now exist as there are curve patterns or qualities stored. Theparameters—amplification or scaling, center frequency and quality—of theshifted and scaled curve pattern for which the smallest error value hasbeen determined are now selected as filter parameters.

[0032] The frequency response determined in this way for the firstequalizer is added to the measured frequency response. These same worksteps are then carried out for ascertaining the filter parameters of thesecond equalizer; here then, the measured frequency response of thepassenger compartment is not taken as a basis, but rather the frequencyresponse of the passenger compartment filtered by the first equalizer.

What is claimed is:
 1. A method for automatically adjusting the filterparameters—center frequency, quality and amplification or attenuation—ofat least one digital equalizer (6,7) which is a component of areproduction device (1) for audio signals in a vehicle passengercompartment, wherein first of all, the acoustical frequency response ofthe passenger compartment is ascertained, the inadequacies in theacoustics of the passenger compartment in the form of local maxima andminima in the measured frequency response are then determined, and thefilter parameters are thereupon adjusted automatically so that at leasta portion of these inadequacies is compensated for.
 2. The method asrecited in claim 1, wherein the acoustical frequency response of thepassenger compartment is ascertained by triggering loudspeaker device(4) of the reproduction device (1) in succession by bandpass noisesignals having different center frequencies, the frequency bands,adjusted in each case in the form of a bandpass noise signal, coveringthe entire audio spectrum, and ascertaining the frequency response inthe form of frequency measuring points for the individual frequencybands, the sound level of the signal which, in this case, is emitted bythe loudspeaker device (4) into the passenger compartment beingdetermined as the frequency measuring point for a specific frequencyband.
 3. The method as recited in claim 2, wherein the bandpass noisesignals for ascertaining the acoustical frequency response of thepassenger compartment are generated with the aid of the equalizer (6,7), in that a noise signal is supplied-to the equalizer (6, 7), and thefilter parameters are adjusted so that a bandpass characteristic havinga narrow bandwidth at a predefined center frequency results for theequalizer (6, 7).
 4. The method as recited in one of claims 1 through 3,a plurality of normalized curve patterns of different quality beingstored for the automatic adjustment of the filter parameters of anequalizer (6, 7), wherein for each curve pattern and each local maximumdetermined in the measured frequency response the center frequency ofthe curve pattern is shifted to the local maximum, an attenuation isdetermined by scaling the curve pattern to the level of this localmaximum, the filter corresponding to this scaled curve pattern is usedon the measured frequency response, and the deviation of the resultingfrequency response from a target frequency response is determined, sothat for each potential center frequency of the equalizer (6, 7), asmany error values for the deviation from the target frequency responseexist as there are curve patterns or qualities stored, and the filterparameters—center frequency, attenuation and quality—of that curvepattern which has led to the smallest error value are taken as the basisfor the automatic adjustment of the equalizer (6, 7).
 5. The method asrecited in claim 4, wherein the individual deviations are weighted whendetermining the deviation of a filtered frequency response from thetarget frequency response.
 6. The method as recited in claim 5, whereinpositive individual deviations are weighted more strongly than negativeindividual deviations.
 7. The method as recited in claim 5, whereinpsychoacoustically critical frequency ranges are weighted more stronglythan psychoacoustically uncritical frequency ranges.
 8. The method asrecited in one of claims 4 through 7, wherein the level of the localmaximum or the resonance corresponding to it is taken into account whendetermining the deviation of a filtered frequency response from thetarget frequency response, so that resonances which are narrow and highcompared to wider, less high resonances are preferably eliminated. 9.The method as recited in one of claims 4 through 8, the filterparameters of at least two digital equalizers (6, 7) being adjustedautomatically, wherein the filter parameters of the individualequalizers (6, 7) are determined in succession, in that, in each case,prior to determining the filter parameters of one equalizer (6, 7), theequalizer(s) (6, 7) adjusted before are used on the measured frequencyresponse.
 10. A reproduction device (1) for audio signals forimplementing a method as recited in one of claims 1 through 9, having aloudspeaker device (4) and having an audio processor (5) that includesat least one digital equalizer (6, 7), is arranged in the signal pathbetween at least one signal source (2, 3) and the loudspeaker device(4), and is connected to a control processor (8) via a control bus (9),wherein a noise generator (10) is provided, via which a noise signal canbe supplied to the equalizer (6, 7); the control processor (8) includesmeans, via which the filter parameters are adjustable so that theequalizer (6, 7) has a bandpass characteristic with a narrow bandwidth,the center frequency being variable over the audio spectrum; at leastone microphone (11) having evaluation means (12) is provided fordetecting the signal emitted by the loudspeaker device (4) into thepassenger compartment and for determining the frequency response; andthe control processor (8) includes means via which the filter parametersare adjustable, taking into account the measured frequency response. 11.The reproduction device as recited in claim 10, wherein the noisegenerator (10) is implemented in the audio processor (5).
 12. Thereproduction device (1) as recited in claim 10, wherein the noisegenerator is implemented in the form of an additional external signalsource.
 13. The reproduction device (1) as recited in one of claims 10through 12, wherein the evaluation means (12) for evaluating the signalsensed by the microphone (11) includes means for the amplification,logarithmation and rectification of the signal.